Barkhausen oscillation elimination means



Nov. 24, 1964 J. P. HOlCE ETAL 3,1

BARKHAUSEN OSCILLATION ELIMINATION MEANS Filed July 24, 1962 Fig 3 IN VE/V TORS Jam P; Haiw wrge L. Angwsm mM Q-W AND/nay United rates atent 3,158,815 BARZLHAUSEN {)SClLLA GN JUMINATKGN MEANS john P. Heine, Albuquerque, N. Eden, and George L. Anderson, Boulder, (3619., assignors, by mesne assi ments, to United States of America as represented by the United States Atomic Energy Commission Filed July 24, 1962, Ser. No. 212,175 2 Claims. (1. 328-215) This invention relates generally to a circuit combination designed to eliminate a specific type of oscillatory effect known as electron or Barkhausen oscillation, within multielement vacuum tubes of the type employed as the active element in electronic circuits.

The invention is of particular importance because of the improvement realized thereby in the performance of blocking oscillators in radar systems and hence it will be discussed, for illustrative purposes, in that context. It will be understood, however, that its principle of operation is inherently adaptable to other circuits which may differ in structure, mode of operation, and function from the blocking oscillator. in order to understand the manner in which the circuit combination of this invention functions to eliminate unwanted electron oscillation as, for example, in the above-referenced radar system, it will be helpful to examine first in some detail the nature of such oscillation.

Electron oscillation, as the term is used in this application, refers to an extremely high frequency oscillation of electrons about the control grid of a triode or other multielement vacuum tube which may occur when such grid is operated at a moderately high positive pot ntial and the anode of such triode is at Zero or a slightly negative potential. Such electron oscillation has been found to ccur at a particular phase in the cyclic operation of electronic circuits such as blocking oscillators and, in particular, the triggered monostable type of blocking oscillator. Of course, it should be understood that electron oscillation can be an essential feature of the operation of certain varieties of oscillator circuits but in the context in which the present invention will be described, although not L mited thereto, such electron oscillation is highly undesirable, since it introduces false indications which must be eliminated.

characte istically, the three-element vacuum tube in a blocking oscillator together with its associated inductive and capacitive circuits generates a fast rising current pulse with which are associated a rapidly rising grid voltage and a rapidly falling anode voltage. It will be understood that two time-varying electric fields exist within the vacuum tube of the oscillator; one between grid and cathode, the other between grid and anode, the polarity of such fields being dependent upon the relative potential of grid, cathode and anode. It will be found that during what is sometimes called the main part of the pulse of the monostable blocking oscillator, i.e., the period of time which falls between the rise time and the recovery interval, a tube saturation condition exists such that the anode may be driven considerably negative with respect to the grid. At such time, the above-referenced time-varying.

the generation of the electron oscillation with which we are concerned.

in general, electrons leaving the cathode of the vacuum tube are accelerated toward the grid owing to its positive potential with respect to the cathode. Those electrons passing through the grid mesh will be retarded by the reversed polarity of the grid-to-anode field. If such electrons should leave the cathode at the beginning of the negative half cycle of the superimposed alternating grid voltage and if, in addition, the cathode-to-grid transit time is substantially equivalent to the time of this negative half cycle, then the acceleration which the electron would otherwise experience in approaching the grid is reduced. As the electron passes through the grid mesh, the brak ing force exerted upon it is further increased due to the now reversed polarity of the superimposed alternating grid voltage. The electron may now have lost sutficient energy to prevent it from ever reaching the plate and to cause it to reverse direction and return toward the grid. The continued coincidence of electron transit-time and half-cycle time duration of the superimposed alternating current may produce a similar reversal in direction when the electron again passes through the grid meshes in the direction of the cathode. In this manner a damped electron oscillation may be set up about the grid since each such electron loses an increasing amount of energy through the electric field in which it is passing. Ultimately these electrons will be swept away by some extraneous process such as an interception by the grid. 7

It may now be appreciated that, with a superimposed high frequency voltage on the grid of the vacuum tube, the electron oscillation described above cannot occur except when the control grid is positive with respect to the anode. The inventors have recognized therefore that the most effective means of eliminating this oscillation lies in the prevention of such potential difference during a specified portion of tube operation.

it is significant to note that electron oscillation occurs at the frequency of the superimposed alternating current, if at all. The shorter the transit time of an electron between one vacuum tube electrode and another the higher the frequency of the electron oscillation. This transit tirne in turn is found to be dependent upon the potential difference existing between electrodes and the distance between them. It should now be understood that the phenomenon of electron oscillation becomes a function of the time-varying electric fields existing within a multielement vacuum tube irrespective of the manner in which these fields are produced.

Suppose now that the electron oscillation described occurs in a blocking oscillator of the triggered monostable variety commonly employed in radar systems. Such a blocking oscillator is designed to generate a fast rising current pulse at a predetermined repetition frequency. However, in radar systems typically operating at frequencies between 509 and 4000 megacycles, any spurious oscillation may cause false echo indications. Electron oscillation in the blocking oscillator has been found to introduce such false echo indications at various high frequencies in the above megacycle range. The circuit combination described hereinafter eliminates electron oscillation within the oscillator and, in consequence, the false indications it produces in a radar system.

The inventors have investigated various means for eliminating the electron or Barkhausen oscillation found to exist in the vacuum tube of a blocking oscillator such as described above. It was found, for instance, that. a permanent magnet placed perpendicularly :across the tube would eliminate electron oscillation completely if a magnetic field of between 308 and 500 gauss could be maintained. The resultant magnetic field accelerates electrons toward the anode strongly enough to overcome the energy Patented Nov. 24, 1964 losses productive of electron oscillation. However, such a magnet has to be properly oriented for maximum efiiciency. Furthermore, demagnetization effects may result fronr long term operation or under the influence of mechanical shock.

. Other prior art meanS of eliminating electron oscillation considered by the inventors include changing the value of the grid resistance, modifying the turns ratio of the pulse'tr'ansformer and providing a series diode in the grid circuit. These, circuit modifications tend to reduce or eliminate the Barkhausen oscillation but only at the expenseof serious degradation of the blocking oscillator output pulse to a degree intolerable under the circumstances such as hereinafter described.

It is thus a general object of this invention to eliminate undesirable electron oscillation within a multielectrode vacuum tube forming the active element in an electronic circuit.

It is a further object of this invention to eliminate undesirable electron oscillation within a multielectrode vacuum tube forming the active element of an electronic circuit without substantial degradation of the output of such circuit. e 7

It is a more specific object of this invention to provide in" a blocking oscillator circuit a modification adapted to eliminate undesirable electron oscillation in said blocking oscillator. It is yet another object to provide in a blocking oscillator'a circuit modification adapted to eliminatefrom said blocking oscillator undesirable electron oscillation in a manner substantiallyindependent of variations in tube temperature.

It is'still another object irra' radar system employing a triggered monostable blocking oscillatorto eliminate therefrom any false echo pulses resulting from electron oscillation within saidblocking oscillator. I

accordance with this invention, a dropping resistor is placed in serieswith the control grid of a rnultielernent vacuum tube functioning as the active element of an electronic circuit, and a diode is connected between the anode of such vacuum tube and the series dropping resistor. The diode is oriented such that its forward bias condition occurs when the control grid tends to become positive with respect to the anode. During this period of diode conduction a finite negative grid potential with respect to the anode is established, the magnitude of which Cir . vided to tube 10 by means of cathode resistor 14 connected between cathode 12 and ground.

Associated with tube 10 is pulse transformer 36 which includes anode winding 17, grid winding 18, and output winding 1%. Anode winding i7 is connected between anode 13 of tube ill and positive terminal through V series resistor 21. Grid winding 18 is connected between control grid 11 and trigger input terminal 22through series dropping resistor 23 and through the. parallel network of blocking capacitor 25 and grid'resistor 26. Grid 1 former 16 may be connected in a typical radar system to a video amplifier (not shown). 7

The characteristics of pulse transformer 16 largely determine the output waveshape produced by the blocking oscillator. In order to develop a high current rectangular pulse, it is desirable to maintain a minimum of leakage inductance which in turn requires a large coefficient of coupling and as few turns as possible.

The operation of this circuit may be initiated by a positive-going external trigger pulse applied to control grid llthrough trigger input 22. A. grid-to-ground resisfor 27 is shown in dotted lines to indicate that this trigger pulse is typically furnished from a low impedance source such as a cathode follower in which case resistor 27 would furnish the necessary cathode-to-ground return. To prevent any negative overshoot of the positive trigger pulse, diode 24 is connected between trigger input terminal 22 and ground, 1

is dependent upon the voltage drop developed across the drOPl Bg resistor while grid current is flowing. This finite negative grid potential eliminates the possibility of unwanted electron oscillation. The resistor-diode circuit'combination of this. invention referenced above may be employed in a blocking oscillator circuit wherein the anode of a vacuum tube is directly connected to the anode windingiof a pulse transformer and the grid of said vacutube is directly connected to the grid winding of said 'pulse transformer. In this configuration the dropping V resistor is connected in series between the grid and the v glld Wllldirig while the diode is connected between the anode and grid windings of the pulse transformer. When the vacuum tube is operated at saturation conditions, a

small potential difference will be maintained between the control grid and the anode, its magnitude determined a by the size of the dropping resistor and the grid current flowing at saturation.

Other objects and advantages of the present invention will' be apparent from the following specification taken in connection with the drawings made a part hereof and the description. of a presently preferred embodiment.

vFIG. 1 is a detailed circuit diagram of a triggered monostable blocking oscillator circuit incorporating the circuit combination of this invention;

FIG. 2 illustrates typical grid and anode voltage wave shapes; during. a cycle of operation of a conventional triggered blocking oscillator; and

516.3 illustrates typical gridrand anode yoltage wave- During tube conduction the anode circuit of this oscillator is in eflfect a powersource of measurable reactance,

which is composed of the small leakage inductance, and

stray capacitance of pulse transformer 16 and other com- 7 7' ponents including the anode-cathode impedance of the tube. Where there is little resistance to dissipate or damp out the shock of abrupt conduction for these reactive elements, the anode voltage generally oscillates ing the circuit with resistor 28 connected across anode winding 17. The ringing only appears after the output pulse has been developed. Capacitor 29, connected becurs.

series dropping resistor 23, as will be hereafter explained,

tween the B-lside of anode winding 17 and ground, in combination with serie's'resistor 21, connected between B-lterminal 243 and capacitor 29, serve as a decoupling network from the 18+ supply. 7

A diode 39 is connected between anode winding 17 and grid winding 18 of pulse transformer 16. Diode 3i) 1 is oriented such that .it will begin to conduct only when a the positive potential attits grid winding connection are ceeds the positive potential at its anode winding connection or in othervwor ds when its forward bias condition oc- It is the function of diode 30 in combination'with cillator circuit which may be regarded as identical to the circuit of FIG. 1, assuming the removal therefrom of series resistor 23 and diode 3t). In that event, grid winding 18 of pulse transformer 16 will become directly connected to grid 11 of tube 1;). In examining the operation of such a conventional blocking oscillator circuit, we are primarily concerned to consider the grid and anode voltage waveforms during a cycle of operation. For this purpose reference should be made of FIG. 2 wherein:

E =D.C. battery potential e =transient anode voltage E =quiescent grid voltage e =transient grid voltage E =grid voltage at cutofi' Under the above assumption, when tube 19 is in a quiescent condition, that is, with no trigger pulse present, a very small anode current flows to bias control grid 11 below cutofi. When a positive pulse is applied to trigger input terminal 22, this pulse is transmitted to control grid 11 through the RC network of grid resistor 26 and capacitor 25 and through grid winding 18. This pulse drives control grid 11 above cutoff and causes an increased anode current to flow through anode winding 17, thus producing a decrease in anode voltage. Because of the regenerative effect provided through the feedback network including anode winding 17 and grid winding 18, the grid voltage and anode current will rise very rapidly depending upon the gain of tube 16. In FIG. 2, it is seen that in the quiescent condition of tube It), the anode voltage assumes the value E and the grid voltage as sumes the value E at some value below the cutoff level E The effect of the trigger pulse is seen as a voltage spike or pip 32 on the rising edge of grid voltage waveform e and a simultaneous pip 33 on the falling anode voltage waveform e During a certain period of time, generally called the main part of the pulse, the anode voltage and grid voltage of the blocking oscillator are both held fairly constant due to the linearly changing magnetizing current in transformer 16. During this period, as shown in FIG. 2, the substantially linear value of grid voltage e (portion 34) may often exceed the corresponding linear value of anode voltage 2,, (portion 35) with respect to ground. Such a condition has been observed on a twochannel oscilloscope having input probes permitting simultaneous observation of the grid and anode voltages of the tube of a conventional blocking oscillator with respect to ground. 1

The linear portion of the grid and anode waveforms persists until the negative charge on capacitor 25 has risen sufiiciently to oppose the voltage on grid 11. Capacitor 25 then begins to discharge and the voltage on grid 11 begins to decrease. A regenerative efiect again takes place and the grid voltage now continues to decrease rapidly to a point below cutofi and simultaneously the anode voltage rises rapidly towards the 13+ potential. It is noted that the anode voltage actually rises above the 3+ potential, this portion of the waveshape resulting from release of residual transformer power. It is at this point that ringing could occur if damping resistor 28 were not connected across anode winding 17. Ultimately both anode and grid voltages return to their original quiescent values to await the arrival of another trigger pulse.

As has been previously noted, it is during the portion of the grid and anode voltage waveforms (references 34 and 35 respectively) at which the grid voltage exceeds the anode voltage that the undesirable electron or Barkhausen oscillation may occur. In fact, under laboratory conditions it was clearly found to em'st. The explanation for the occurrence of this oscillation and the mechanism thereof has already been explained.

Returning now to the specific circuit of this invention as exemplified in FIG. 1, we may consider the improved operation achieved by insertion of series dropping resistor 23 and diode 3%. In this connection, reference should be made to FIG. 3 which illustrates grid and anode voltage waveforms of the modified blocking oscillator whose various values are labeled as in FIG. 2.

Initially, as seen from the waveforms in FIG. 3, the operation of the modified blocking oscillator circuit is similar to that of the unmodified version. As before, a trigger pulse applied at trigger input 22 causes the voltage on grid 11 to rise positively with a corresponding decrease in the voltage at anode 13. Again there appears a voltage spike or pip 37 on the rising edge of the grid voltage waveform and a corresponding voltage spike or pip 38 on the falling edge of the anode waveform. During the rise period of grid voltage e diode 30 behaves as a high impedance due to the manner of its bias and thereby does not affect the output waveform. However, at the instant anode voltage e tends to go negative with respect to grid voltage e diode 30 will begin to conduct. Therefore, it behaves thereafter as a low impedance or, eifectively, as a closed switch. The grid current which is being drawn through dropping resistor 23 develops a voltage drop across this resistor such that control grid 11 has a finite negative potential with respect to the terminal of diode 30 which is coincident with resistor 23. This has the important eiiect of clamping control grid 11 at a potential value less than that of anode 10 and maintaining such potential difierence during the linear portion of the pulse. It will be noted that linear portion 39 of grid waveform e is now maintained at a value less than corresponding linear portion 40 of anode waveform e The value chosen for resistor 23 is clearly dependent upon the minimum voltage separation desired between anode 10 and grid 11. An upper limiting value is usually imposed upon resistor 23 by the maximum permissible effect which the voltage drop across it may have upon the amplitude of the resulting output pulse. For example, with a minimum separation of 5 volts between e and e there is found to be no noticeable change in the amplitude of the output pulse.

The characteristics of diode 30 are such that there is always some current leakage under a sufliciently high grid current. If sufliciently large, such current leakage could effectively create a free running blocking oscillator and, therefore, the desirable diode specifications should state the maimum permissible leakage current under the maximum peak voltage which may occur in the circuit.

FIG. 3 clearly illustrates the desired voltage separation between e and e Again recalling the basic analysis of the conditions under which electron oscillation may occur, it will be appreciated that this voltage separation efiectively inhibits or prevents the occurrence of such oscillation, which is precisely the effect sought to be produced by the circuit modification of this invention. The grid voltage oscillation superimposed as a result of the reactive elements of the oscillator may still be considered to occur but in the absence of a positive grid to anode voltage condition, will no longer have the undesirable results heretobefore described.

The clamping action of diode 30 in conjunction with resistor 23 has a further advantage in addition to that of substantial elimination of electron oscillation. It will be observed that since the linear portion of the anode and grid waveforms are controlled, the output pulse amplitude is held more nearly constant than would otherwise be the case. Small changes in supply voltage will, in view of the present circuit modification, not affect the clamping action since diode 30 will conduct at the instant control grid 11 seeks to go positive with respect to anode 13.

It should be apparent that the circuit improvement described can be adapted to any situation wherein the operation of a vacuum tube in an electronic circuit is productive of grid and anode waveforms giving rise to resi'stor combination will thus have general application within a variety of electronic circuits and those skilled in the art will appreciatethe' areas in which its use might.

be desirable. 7

What is claimed is: '1. A triggered rnonostable blocking oscillator circuit comprising in combination a multielement vacuum tube having at least an anode, a cathode, and a control grid, inductivefeedback means connected between said anode and said control grid, a source of DC. potential, circuit meansfor supplying said DC. potential to said anode, trigger input means, said control grid being connected to said trigger input means through said" feedback means in seri'es with a parallel resistive-capacitive network, said cathode being connected to ground through a self-biasing resistor, a dropping resistor and a diode connected in series between said control grid and said anode, said diode oriented such that its forward bias condition oc curs when the junction of said diode and said dropping resistor becomes positive with respect to saidanode.

2. A triggered monost'able blocking oscillator circuit comprising in combination'a multielernent vacuum tube having at least an anode, a cathode, and a control grid, 7

a pulse transformer having-at least anode and grid Windings and adapted to establish a positive feedback loop with said vacuum tube, a source ernc. potential, said anode being connected to said sourceof DC. potential through said anode winding, trigger input means, said control grid being connected to said'trigger input means through said grid winding in series with a parallel re sistive-capacitive network, said cathode being connected to ground through a self-biasing resistor, a dropping re sistor connected between said control grid and said grid winding, and a diode connected between said anode and grid windings and oriented suchrthat its forward bias condition occurs when the junction of said diode and said dropping resistor becomes positive with respect to said anode.

References Qit'ed in the file'of'this patent UNITED STATES PATENTS 2,602,896 whiraker nt July 8, 1952 

1. A TRIGGERED MONOSTABLE BLOCKING OSCILLATOR CIRCUIT COMPRISING IN COMBINATION A MULTIELEMENT VACUUM TUBE HAVING AT LEAST AN ANODE, A CATHODE, AND A CONTROL GRID, INDUCTIVE FEEDBACK MEANS CONNECTED BETWEEN SAID ANODE AND SAID CONTROL GRID, A SOURCE OF D.C. POTENTIAL, CIRCUIT MEANS FOR SUPPLYING SAID D.C. POTENTIAL TO SAID ANODE, TRIGGER INPUT MEANS, SAID CONTROL GRID BEING CONNECTED TO SAID TRIGGER INPUT MEANS THROUGH SAID FEEDBACK MEANS IN SERIES WITH A PARALLEL RESISTIVE-CAPACITIVE NETWORK, SAID CATHODE BEING CONNECTED TO GROUND THROUGH A SELF-BIASING RESISTOR, A DROPPING RESISTOR AND A DIODE CONNECTED IN SERIES BETWEEN SAID CONTROL GRID AND SAID ANODE, SAID DIODE ORIENTED SUCH THAT ITS FORWARD BIAS CONDITION OCCURS WHEN THE JUNCTION OF SAID DIODE AND SAID DROPPING RESISTOR BECOMES POSITIVE WITH RESPECT TO SAID ANODE. 